Core D Summary/Abstract: Characterizing groundwater flow is essential for documenting the geochemical processes in an aquifer that affect arsenic (As) so that these findings can be generalized to other settings. The Hydrogeology Core D will provide the tools and expertise for collecting and interpreting a broad range of hydrogeological data in the US and in Bangladesh. At the Massachusetts Military Reserve research site on Cape Cod maintained by the US Geological and the Lot 86 Superfund site at North Carolina State University, Core D will support the collection of sediment cores and reactive-transport modeling of As on behalf of Project 3. In shallow aquifers of the Health Effects of Arsenic Longitudinal Study (HEALS) cohort area in Bangladesh, Core D will also support the collection of sediment cores and dating of groundwater using the 3H-3He method in 50 monitoring wells for comparison with data obtained over a decade ago. In deeper aquifers of the HEALS area, Core D will support additional 3H measurements to document the downward penetration of shallow groundwater. These results, along with water levels and the stable isotopic composition of water, will be targets for modeling the impact of asynchronous irrigation pumping on the redistribution of As in shallow aquifers on behalf of Project 4.